1. Field of the Invention
The present invention relates to a process for electrolyzing an aqueous solution of an alkali metal chloride. More particularly, it relates to a process for producing an alkali metal hydroxide by electrolyzing an aqueous solution of an alkali metal chloride in a low electric power consumption.
2. Description of the Prior Art
As a process for producing an alkali metal hydroxide and chlorine by electrolysis of an aqueous solution of an alkali metal chloride, a diaphragm method has been mainly employed instead of a mercury method in view of the prevention of pollution.
It has been proposed to use an ion exchange membrane in place of asbestos as a diaphragm to produce an alkali metal hydroxide by electrolyzing an aqueous solution of an alkali metal chloride so as to obtain an alkali metal hydroxide having high purity and high concentration.
On the other hand, it has been proposed to save energy in this world. From the viewpoint, it has been required to minimize the cell voltage in such technology.
It has been proposed to reduce cell voltage by improvements in the materials, compositions and configurations of an anode and a cathode and compositions of an ion exchange membrane and a kind of ion exchange group.
In these processes, certain advantages can be considered. However, in most of these processes, the maximum concentration of the alkali metal hydroxide is not so high. In the case of higher concentration over the critical concentration, the cell voltage is seriously increased or the current efficiency is remarkably lowered. The maintenance and durability of the low cell voltage phenomenon have not been satisfactory for an industrial purpose.
It has been proposed to reduce the cell voltage by decreasing the distance between electrodes. However, the decrease of the distance between an anode and a cathode with a cation exchange membrane therein is limited in the following aspects.
One aspect is as follows:
The cell voltage decreases depending upon a decrease of the distance between the electrodes to an average distance between the electrodes of about 3 mm, however, the cell voltage may increase by decreasing further the distance, because of an adhesion of bubbles, and residence of bubbles in a conventional combination of an expanded metal type anode and cathode.
The other aspect is as follows:
A flattening precision of a surface of an electrode results in the limitation. The flattening precision of the surface of the electrode depends upon size and the limitation is usually considered to be .+-.1 mm. In order to set a cation exchange membrane without damage, an average distance between electrodes of 1 mm or more is usually required. It is possible to decrease further the average distance between electrodes by improving the flattening precision of the surface of the electrode or providing a mechanism for absorbing the unevenness of the surface of the electrode in view of technology. However, serious labours and precise processing are required. Therefore, it is not advantageous in an industrial operation.
The inventors have studied to overcome these disadvantages and have found that the first problem can be solved by forming a thin porous layer on the cation exchange membrane and the second problem can be solved to attain a low cell voltage without increasing the flattening precision and decreasing an average distance between electrodes.